This invention relates to an apparatus and method for fabricating semiconductor thin films. More specifically, the invention relates to the conversion of layers of copper and indium into thin films of the semiconductor copper indium diselenide semiconductor, CuInSe.sub.2. CuInSe.sub.2 thin films are useful for photovoltaic solar cells device such as described by R. W. Birkmire et al. in Solar Cells, Volume 16, aperes 419-427 (1986).
CuInSe.sub.2 films have been formed by several different deposition methods including molecular beam epitaxy, physical vapor deposition, sputtering and electrodeposition. An alternate technique for forming CuInSe.sub.2 films is described by Kapur et al in U.S. Pat. No. 4,581,108 and by Wieting et al in U.S. Pat. No. 4,612,411. The technique, known in the art as selenization, is carried out in two steps: sequential deposition of copper and indium layers on a suitable substrate; followed by reaction at approximately 400.degree. C. with a gaseous selenium compound or species. The selenium is introduced as either elemental selenium vapor entrained in an inert carrier gas or hydrogen at atmospheric pressure, or as the compound, hydrogen selenide--H.sub.2 Se. The Wieting et al patent describes use of the selenization process for fabricating CuInSe.sub.2 photovoltaic devices. However, selenization using currently known apparatus and methods have a number of disadvantages which limit the applicability of selenization for large scale production of semiconductor thin films and solar cells.
When elemental selenium is used as the source for selenium, the following must be dealt with:
The entire reactor system, including source containers for the elemental selenium, must be maintained at temperatures at least above the melting point of selenium (about 200.degree. C.) and more likely at least 400.degree. C. This makes it difficult to control the rate of delivery of selenium to the reacting substrates, and leads to problems with the selenium depositing on surfaces which are only slightly cooler than the nominal system temperature.
An inert carrier gas must be used to transport the selenium species to the substrate. This is generally accomplished via forced convection in a once through reactor design. Hence, selenium species which do not react with the copper and indium on the substrate after the one and only pass through the reactor are wasted.
When hydrogen selenide is used as the source of selenium, the following must be dealt with:
Hydrogen selenide is an extremely toxic compound. In selenization processes known in the art, only a small portion of the hydrogen selenide which is introduced into a once-through reactor reacts to form CuInSe.sub.2. The vast majority of the hydrogen selenide must be treated in a waste treatment system prior to release of the gas stream to the atmosphere.
Therefore, a disadvantage shared by all presently known selenization reactors is poor utilization of either the selenium or hydrogen selenide source material. This is an inherent characteristic of the once-through design of the reactors used for selenization. The disadvantages and problems associated with the current technology of selenization make development of the selenization process into a commercial scale operation less attractive. It should also be noted that many of the disadvantages noted above for the selenization process are common to other semiconductor film formation reactions and associated reactors. For instance, materials utilization is generally considered to be quite low in many chemical vapor deposition (CVD) and plasma enhanced CVD processes used for the formation of such compounds as gallium arsenide, indium phosphide and cadmium telluride.